Hydroquinone and its derivatives have been used as the preferred developer of image-wise exposed photographic film for a great many years to the point where they have been considered to be an irreducible requirement for an effective developer formulation. Their low cost and chemical properties related to redox potential, activity and long term stability during repetitive use conditions have made hydroquinone synonymous with superior silver halide developer formulation. They have been found to be useful in rapid access developer formulations for developing very high contrast films that exhibit high photographic speed and excellent dot quality. These high contrast films are made possible by including in the film certain hydrazine compounds that serve as infectious development nucleators, augmented by nucleation boosters prepared from amino compounds.
The position of hydroquinone as a preferred developer has recently come under scrutiny and disfavor as a result of a growing realization that these chemicals may present certain ecological and environmental hazards to society. These findings have triggered a surge of research among artisans in the photographic film industry to discover developers that are environmentally benign while also functioning as well as hydroquinones for the development of conventional as well as high contrast film containing nucleators. One compound that has been found to approximate the performance of dihydroxy benzene while remaining environmentally benign is ascorbic acid.
It is known that ascorbate developer pH tends to drop during use making ascorbate developer stability an obstacle to overcome in providing a stable developer that is useful for machine use when large amounts of photographic film are processed. In conventional developers based on hydroquinone the drop in pH due to processing of film is somewhat offset by the increase in pH due to aerial oxidation. Since hydroquinone has a pK.sub.a of 9.9 it is able to provide significant buffering. Since ascorbic acid has a pK.sub.a of 11.4 it does not contribute significant buffering in the useful range of developers (10.0 10.8). Ascorbate based developers decrease in pH when aerially oxidized as well as during development.
When compared to conventional dihydroxybenzene developers, ascorbate-based developers must meet and overcome other challenges in addition to overcoming problems in chemical stability. Ascorbate developers must be able to provide a high contrast product as typically produced in the art by photographic elements and/or hydroquinone developers containing high contrast promoting nucleators and amine boosters. U.S. Pat. No. 4,975,354 first described the use of "booster" technology, and U.S. Pat. No. 4,994,365 describes the use of alkyl ballasted pyridine nucleators as a method to improve image quality with the incorporated boosters. These patents are best represented by the following analog examples of Nucleators A and B and Booster I: ##STR1##
The inclusion of boosters in photographic systems, whether in the developer or in the film elements, presents a new set of problems. When in the developer, they can attack the processor equipment. The drawback of the systems which incorporated the alkanol amine boosters into the film containing the nucleators was the complexity of balancing the nucleator with the boosters to provide good discrimination at low fog or pepper levels while broadening the degree of compatibility with a number of existing rapid access developer systems. U.S. Pat. No. 5,264,323 describes the complications of balancing the hybrid systems which involves both nucleator plus booster.
U.S. Pat. No. 5,264,323 also describes an improved photographic developing solution which is free of dihydroxybenzene developing agents comprising an alkaline aqueous solution containing ascorbic acid, an auxiliary developing agent and a carbonate buffering agent in a concentration of at least 0.5 molar. The patent states that the developing solution exhibits excellent stability with respect to seasoning effects, and provides high speed and contrast combined with a low level of pepper fog and a moderate degree of chemical spread. The developer is used to develop images on an imagewise exposed photographic element or film that contains a hydrazine compound that functions as a nucleating agent and an amino compound that functions as an incorporated booster.
The objective of the present invention is to describe a process of forming a high contrast product cable of good image discrimination as evidenced by good dot production.
A second objective of the invention is to describe a process employing a hydroquinone-free developer solution to provide excellent image discrimination and high contrast, preferably in the absence of "boosters".
A third objective of the invention is to describe a hydroquinone-free developing solution capable of giving excellent results wherein the developer composition provides excellent resistance to aerial oxidation.